The present invention relates generally to semiconductor manufacturing and, more specifically, to the conditioning of polishing pads used for chemical-mechanical polishing (CMP).
Chemical-Mechanical Polishing (CMP) is a key processing technology for fabricating semiconductor chips. Often, after the performance of a processing step, the resulting wafer surface is full of peaks and valleys. Peaks and valleys of subsequent processing steps can build upon one another, creating an uneven surface that may be undesirable for a number of reasons. CMP uses a polishing pad and a slurry of chemically active liquid and abrasive material to grind down the surface of a wafer, thus restoring the planar surface.
In particular, CMP is useful for planarizing intermetal dielectric layers of silicon dioxide or for removing portions of conductive layers within integrated circuit devices. Non-planar dielectric surfaces may interfere with the optical resolution of subsequent photolithography processing steps, making it extremely difficult to print high-resolution lines. The application of a second metal layer over an intermetal dielectric layer having large step heights can result in inadequate metal coverage, and ultimately in an open circuit.
In an exemplary CMP process, the semiconductor wafer is held face down and rotated against a flat polishing pad that has been coated with the slurry. Both the wafer and the pad are typically rotated relative to each other. The abrasive polishing process continues until the surface of the wafer contacting the pad is substantially planar.
The motion of the wafer with respect to the polishing pad and the force applied to hold the wafer against the pad adds mechanical energy to the system that helps remove the wafer surface material. In addition, the process of supplying fresh chemical liquid and removing spent chemical liquid helps remove material from the wafer surface. Uniform removal of material from the surface of the wafer is pursued by adjusting a number of variables, such as the pad velocity with respect to the wafer surface, the force applied between the pad and the wafer, and the slurry composition and flow.
Over time, the initially rough surface of the polishing pad becomes worn and may glaze over due to a build-up of slurry and other deposits on the pad surface. To counteract the glazing and wear, the polishing pad is periodically mechanically scored or xe2x80x9cconditioned.xe2x80x9d Conditioning the pad removes the build-up on the pad and roughens the surface of the pad. Different approaches to conditioning may be required depending on the hardness of the pad surface and the particular slurry used for polishing. Conditioning may be performed by a conditioning apparatus in a discrete conditioning step or during wafer polishing depending on the specific conditioning process and apparatus used.
In one type of conditioning process, a rotating conditioning pad having a diameter much smaller than the diameter of a rotating polishing pad is moved across the polishing pad by, for example, a robotic arm. A number of types of conditioning pads are known in the art. In particular, it is known to use a conditioning pad comprising the same material as the polishing pad, as disclosed in pending U.S. patent application Ser. No. 09/532,170, titled xe2x80x9cPolishing Pad Reconditioning Via Polishing Pad Material As Conditioner,xe2x80x9d filed on Mar. 21, 2000, and assigned to the common assignee of the subject invention.
Measurements have found a direct correlation between the profile of the substrate and the pad polishing the substrate. For example, FIG. 1 shows a plot of the relative thickness of an exemplary polishing pad 10 after the pad has been used in a rotary CMP process over a period of time and has become worn differently in one region as compared with another. More wear is shown in regions 12 and 14 than in edge regions 16 or center region 18. The polishing rate and uniformity of the CMP process may be greatly affected by the characteristics of the polishing pad surface, which can make the slurry more or less effective.
Although conditioning of the polishing pad surface improves polishing uniformity and rates, it has the detrimental effect of removing a quantity of polishing pad material. Uneven wear in the polishing pad may be caused by characteristics of the wafers or the location on the pad of the wafers being polished, by non-uniformities introduced by the polishing tool, or by non-uniform removal of pad material during conditioning. Such uneven wear may adversely affect the useful lifetime of the pad.
Others have described a number of complex methods and apparatus for detecting or calculating changes in wear in the polishing pad and then adjusting the pad conditioning parameters accordingly. For example, U.S. Pat. No. 6,045,434, owned by the common assignee of the subject invention, describes a method whereby a non-intrusive measurement of change in the polishing pad thickness is taken in various locations, and the pad conditioning or polishing tool parameters are adjusted accordingly. The non-intrusive measurement system is described as an array of sensors aligned over the pad surface.
A need remains, therefore, to provide a novel apparatus and method for providing a uniform polishing pad surface without additional, fixed, non-intrusive measurement apparatus aligned over the pad surface.
To meet this and other needs, and in view of its purposes, the present invention provides a conditioning tool that achieves uniform conditioning of a chemical-mechanical polishing pad. The conditioning tool includes a rotary conditioning pad; a lower shaft attached to the conditioning pad; an upper shaft having an upper end and a lower end, the lower end attached to the lower shaft via a flexible coupling; and a motor attached to the upper end of the upper shaft and adapted to rotate the shaft. The tool further includes a mechanism for measuring an angle of the conditioning pad relative to a reference plane. The mechanism for measuring the angle of the conditioning pad may comprise a follower spaced apart from and attached to the conditioning pad that replicates the angle of the conditioning pad, and one or more proximity sensors supported above the follower. Three sensors may be used to determine the coordinates of the plane in which the follower lies.
The conditioning tool may further include a conditioning arm adapted to support the motor and to sweep the motor and attached conditioning pad across the polishing pad. Various control components, such as an element for controlling rotation speed of the motor imparted to the shaft, an element for controlling the radial position of the conditioning arm, and an element for controlling the pressure exerted by the conditioning pad against the polishing pad may be provided. A controller for receiving feedback signals from the proximity sensors and the various control components may also be included. The controller provides control signals to the various control components in response to the feedback signals.
The subject invention also encompasses a chemical-mechanical polishing apparatus comprising a polishing pad, a conditioning tool, and a mechanism for controlling the conditioning parameters of the conditioning tool relative to the polishing pad.
In a method for conditioning a polishing surface of a chemical-mechanical polishing pad in accordance with this invention, a first step comprises measuring a variation in height over the polishing surface using the conditioning tool. Next, the measured variation is analyzed to identify at least a first region in need of greater conditioning and a second region in need of lesser or no conditioning. Finally, the first and second regions are conditioned using at least one conditioning parameter in the first region that is different than in the second region to meet the need identified by the profile of the pad surface.
These steps may be repeated until the pad reaches a predetermined state of planarity. The different conditioning parameter may comprise the pressure exerted by the conditioning tool against the polishing pad, the rotation speed of the conditioning tool, the relative amount of time spent conditioning the polishing pad in the first region as compared to the second region, or a combination of these parameters. The method of the present invention may incorporate using the conditioning tool of the invention.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.